The present invention relates to video data transmission and is applicable in particular, though not necessarily, to video telephony.
There are many applications in which the high speed transmission of video data is required. Demand is expected to rise in particular for video telephony services (including video conferencing) over the next few years. With a view to anticipating this expected rise in demand, and also increased demand for other services which require the transmission of video data, the International Telecommunications Union (ITU) is presently laying down new standards in this area. In particular, the Telecommunication Standardisation Sector of ITU (known as ITU-T) has produced a number of draft recommendations, as a precursor to international standards, relating to the point-to-point transmission of video data (and for example available at internet address xe2x80x9cftp://standards.pictel.comxe2x80x9d).
ITU-T recommendation H.324 defines a terminal for low bit rate multimedia communication. H.324 terminals may carry real-time voice, data and video, or any combination thereof and may be integrated into personal computers or implemented in stand-alone devices such as video telephones. The H.324 recommendation incorporates a number of other ITU-T recommendations and is intended to maximise the interworking of terminals which will meet the standard which will result from this recommendation.
The recommendations which are incorporated into H.324 include:
recommendation H.261 which specifies inter alia the processing and coding of colour format images comprising luminance (Y) and chrominance (U,V) components;
recommendation H.263 which specifies a coded representation (video codec) which can be used for compressing and decompressing the moving picture component of an audio-visual transmission (including H.261 format image data) and which meets the low bit rate requirement; and
recommendation H.245 which specifies the protocol for exchanging audio-visual and data capabilities between two connected terminals and defines the properties of the transmission channel and the properties of the transmitted data.
There is an ongoing desire to reduce still further the bit-rate required for video data transmission in order to increase transmission channel capacity and to reduce data processing requirements. The present invention provides for such a reduction by transmitting only the luminance (Y), i.e. black and white, component of a video signal, and omitting the chrominance (U,V), i.e. colour, components when a receiving terminal has the ability, or is set, to display only black and white images.
According to a first aspect of the present invention there is provided a method of transmitting video data from a first to a second terminal over a transmission channel, the method comprising the steps of:
providing at said first terminal a video signal having luminance (Y) and chrominance components (U,V);
in the event that chrominance components are not required by the second terminal, transmitting from the second terminal to the first terminal a control message indicating that only the luminance (Y) component of said video signal should be transmitted to the second terminal; and
subsequent to receipt of said control message at the first terminal, transmitting from the first terminal to the second terminal only the luminance component (Y) of the video signal.
The present invention results at least in part from the realisation that, where a receiving terminal is only capable of displaying black and white images, chrominance component signals (U,V) sent to the receiving terminal represent redundant information. By omitting the chrominance components from the transmitted video data, valuable transmission capacity can be freed for other uses.
Preferably, the method comprises encoding said video data at said first terminal, prior to transmission, using a video encoder. More preferably, the step of encoding provides for the compression of the video data. Encoded data is decoded at the second terminal by a video decoder.
In a first embodiment of the present invention, the luminance (Y) and chrominance components (U,V) of the video signal are provided to the video encoder. However, the video encoder encodes only the luminance component (Y). No chrominance signals are encoded or transmitted.
In a second embodiment of the present invention, both chrominance and luminance components (Y,U,V) of the video signal are provided to the video encoder. The video encoder then encodes the chrominance components (U,V) as if they were null signals. Where the data is encoded with a technique such as variable length coding, the null data can be greatly compressed and does not significantly add to the volume of transmitted video data. The video data received at the second terminal is processed by a video decoder to provide the luminance component (Y) and the null chrominance components (U,V). The latter are then discarded and only the luminance component is retained. An advantage of this embodiment is that, whilst the video encoder at the first terminal requires to recognise that luminance only transmission is required, the decoder at the second terminal does not.
In a third embodiment of the present invention, the chrominance components (U,V) are replaced by null signals prior to providing them to the video encoder. The encoder then encodes the null chrominance signals, together with the luminance component (Y), and transmits the encoded data. This allows an otherwise standard encoder to be used. Similarly, a standard decoder may be used at the second terminal.
Preferably, the method comprises using a colour video camera to provide said video signal to the first terminal, and providing the transmitted luminance component to a black and white display coupled to the second terminal.
The method of the present invention may comprise the step of transmitting a further control message, from the second to the first terminal, subsequent to the transmission of the first mentioned control message, indicating that the luminance (Y) and chrominance (U,V) components of said video signal should be transmitted to the second terminal. Thereafter, all of these components are transmitted. This may be appropriate, where the second terminal has both black and white and colour display capabilities, but where there is from time to time a need to reduce the transmission bandwidth.
According to a second aspect of the present invention there is provided apparatus for transmitting video data over a transmission channel, the apparatus comprising:
a first terminal arranged to receive a video signal having luminance (Y) and chrominance components (U,V); and
a second terminal arranged to transmit therefrom, to said first terminal, a control message indicating that only the luminance (Y) component of said video signal should be transmitted to the second terminal,
said first terminal being further arranged to transmit to the second terminal only the luminance component (Y) of the video signal, subsequent to receipt of said control message.
According to a third aspect of the present invention there is provided a receiving terminal for use in the video data transmission apparatus of the above second aspect of the present invention, the terminal being arranged to transmit therefrom, to a remote terminal, a control message indicating that only the luminance (Y) component of said video signal should be transmitted from the remote terminal to the receiving terminal.
According to a fourth aspect of the present invention there is provided a transmitting terminal for use in the video data transmission apparatus of the above second aspect of the present invention, the terminal being arranged to receive a video signal having luminance (Y) and chrominance components (U,V), to receive a control message, from a remote terminal, indicating that only the luminance (Y) component of said video signal should be transmitted from the transmitting terminal to the remote terminal, said transmitting terminal being further arranged to transmit to the remote terminal only the luminance component (Y) of the video signal, subsequent to receipt of said control message.
According to a fifth aspect of the present invention there is provided a video data transmission unit comprising, in combination, a terminal according to the above third aspect of the present invention and a terminal according to the above fourth aspect of the present invention.
According to a sixth aspect of the present invention there is provided a method of transmitting video data between a first and a second terminal over a transmission channel, the method comprising the steps of:
exchanging flags between the first and second terminals to notify each terminal of the capability of the other terminal to transmit and receive image data with and without chrominance components;
transmitting video data from the first to the second terminal, and/or vice versa, on the basis of the exchanged flags.